No abstract

The resistance R of the 2DEG on the vicinal Si surface shows unusual behavior which is very different from that in Si (100) MOSFET's studied earlier. The low-temperature crossover from dR/dT<0 ("insulator") to dR/dT>0 ("metal") occurs at a low resistance of R(c)square approximately 0.04xh/e2. This crossover, which we attribute to the existence of a narrow impurity band at the interface, is accompanied by a distinct hysteresis in the resistance. At higher temperatures, another change in the sign of dR/dT is seen. We describe it by temperature dependent impurity scattering of the 2DEG near the transition from the degenerate to nondegenerate state.

We show that shot noise can be used for studies of hopping and resonant tunnelling between localised electron states. In hopping via several states, shot noise is seen to be suppressed compared with its classical Poisson value S I ¼ 2eI (I is the average current) and the suppression depends on the distribution of the barriers between the localised states. In resonant tunnelling through a single impurity an enhancement of shot noise is observed. It has been established, both theoretically and experimentally, that a considerable increase of noise occurs due to Coulomb interaction between two resonant tunnelling channels.

The behavior of emitted LA phonons when an electric field is applied to b-doped (001) GaAs is investigated by the time-of-Sight method. The characteristics show peculiar features due to speci6c interaction processes involved, i.e. , direct emission by heated carriers as well as resonance absorption of nonequilibrium LA phonons besides decay and conversion processes of high-energy phonons. The latter contributions are in accordance with Monte Carlo simulations.The time-of-Bight method does not only allow us to investigate the kinetics of phonon Buxes (see for instance Re&. 1 -3), but also to draw conclusions with respect to their emission by hot carriers. 4 s These reports describe electrons confined in GaAs/Al Gaz As quantum wells heated by applied electric fields and the phonons emitted from these sources and propagating through the substrate to the bolometer. In the present paper such investigations are performed for b-doped GaAs layers, in which the electrons are confined to V-shaped quantum wells in thermal equi&ibrium, and attention is essentially paid to comparatively weak yet heating electric fields. By fabricating a very sensitive bolometer it has become possible to study the role of various electron-phonon interaction processes in contrast to strong electric fields, for which, on account of a high emission rate of optic phonons, the decay and conversion processes of phonons play the dominant role as exhibited in our recent publication. The investigation of the LA mode as a function of carrier heating can be performed in contrast to Re&. 4 -7. It should be emphasized that this is the first time to our knowledge that the LA mode along the defocusing (001) direction has been investigated in detail, up to now it was merely detected in GaAs (Ref. 1) and in Si, respectively, by thermalization of photoexcited electron-hole pairs. The investigation concerns GaAs grown on 3.4 nun thick substrates by molecular beam epitaxy (MBE) and containing two b-shaped layers in a distance of 100 nm doped with 1.35 and 1.2 x 10~2 cm Si atoms, respectively, as earlier described in Ref. 7. Contacts of Au:Ge were alloyed in a distance of 250 pm. In contrast to the measurements in Ref. 7 now an In bolometer was evaporated meanderlike on the backside of the substrate in order to enhance the sensitivity. The samples were immersed in liquid He of 2 K. Rectangular voltage pulseswere applied with a duration between 30 and 100 ns and a repetition rate up to 500 cps.Concerning the donor concentration, the average distance within a b plane is small enough to form quantum wells. While in the absence of an external excitation the electrons are mainly in the ground level as well as the 6rst excited level and concentrated close to the dopant layer, therefore, by applied electric fields the electrons in the 30 c5 CD 20-CL CL TA F = 37V/crn 10-CL C O C 0 CL I 0.5 1.0 I 1.5 time [10 &j FIG. 1. Time-of-Sight spectrum of nonequilibrium phonons at 2 K for an applied field strength of 37 V/em and a pulse duration of 30 ns. The bolometer signals are ...

We show that shot noise is a useful tool in studies of electron transport in hopping and resonant tunnelling between localised states. In resonant tunnelling through a single impurity we observe an enhancement of shot noise caused by Coulomb interaction between two resonant channels. In hopping along chains of localised states, shot noise is shown to be sensitive to the distribution of the barriers between the states and the Coulomb interaction between hopping electrons.